1. Field of the Invention
The present invention relates to band rejection filters, and particularly relates to a band rejection filter having a ladder circuit configuration including a plurality of elastic wave resonators.
2. Description of the Related Art
In recent years, broadcasting service for mobile devices for digital terrestrial television broadcasting has been started in various countries including Japan, and practical use of such broadcasting service has been discussed in many countries. Under this circumstance, cellular phones having a function of receiving broadcasting service for mobile devices for digital terrestrial television broadcasting have been put into practical use. The cellular phones having the function of receiving broadcasting service for mobile devices for digital terrestrial television broadcasting include TV tuners used to receive the broadcasting service for mobile devices. In such a cellular phone, there arise problems in that, if a transmission signal of the cellular phone and a broadcasting signal of the broadcasting service for mobile devices interfere with each other, sensitivity for receiving the broadcasting service for mobile devices is deteriorated and a TV tuner for receiving the broadcasting service for mobile devices malfunctions.
Therefore, such a cellular phone includes a band rejection filter used to prevent interference between the transmission signal of the cellular phone and the broadcasting signal of the broadcasting service for mobile devices by preventing the transmission signal of the cellular phone from being transmitted to the TV tuner used to receive the broadcasting service for mobile devices. The band rejection filter has a function of allowing the broadcasting signal of the broadcasting service for mobile devices to be transmitted to the TV tuner used to receive the broadcasting service for mobile devices and removing the transmission signal of the cellular phone, and is disposed between an antenna and the TV tuner used to receive the broadcasting service for mobile devices in the cellular phone.
The band rejection filter should have a filter characteristic in which a passband which allows transmission signals of the broadcasting service for mobile devices to pass and an attenuation band which removes transmission signals of the cellular phone are included. In the broadcasting service for mobile devices for digital terrestrial television broadcasting in Japan, a band for broadcasting signals corresponds to a UHF band (470 MHz to 770 MHz). On the other hand, a band for transmission signals for cellular phones corresponds to a band in the vicinity of 800 MHz to 900 MHz. Therefore, band rejection filters included in the cellular phones having the function of receiving the broadcasting service for mobile devices in Japan include a passband corresponding to the UHF band (470 MHz to 770 MHz) which is the band for the broadcasting signals and an attenuation band corresponding to a band in the vicinity of 800 MHz to 900 MHz which is the band for the transmission signals for the cellular phones.
In such a band rejection filter, in order to appropriately remove the transmission signals of the cellular phones, high sharpness of a filter characteristic is required between an end of the passband which allows the broadcasting signals of the broadcasting service for mobile devices to pass and the attenuation band which removes the transmission signals of the cellular phones.
In general, as a filter having a passband and an attenuation band, a filter device having a ladder circuit configuration is known. For example, Japanese Patent No. 3827232 and Japanese Unexamined Patent Application Publication No. 10-65490 disclose various filters having respective ladder circuit configurations utilizing elastic waves. FIG. 29 shows a filter device 101 having a ladder circuit configuration disclosed in Japanese Patent No. 3827232. In the filter device 101, a series arm which connects an input terminal 102 to an output terminal 103 includes a plurality of series arm resonators 111 to 113 which are connected to one another in series. Furthermore, a plurality of parallel arms are disposed between the series arm and the ground potential. The plurality of parallel arms include respective parallel arm resonators 114 to 117.
In the filter device 101, a capacitor 118 is connected to the parallel arm resonator 114 in parallel among the plurality of parallel arm resonators 114 to 117. Therefore, an electromechanical coupling coefficient of the parallel arm resonator 114 is smaller than those of the parallel arm resonators 115 to 117. As a result, sharpness of the filter characteristic at ends of a passband is enhanced.
Furthermore, Japanese Patent No. 3827232 discloses the following two methods as other methods for making one electromechanical coupling coefficient different from another electromechanical coupling coefficient:                (1) a method for setting different standard film thicknesses (h/λ) of IDTs (Interdigital Transducers) included in resonators; and        (2) a method for setting different duty ratios of the IDTs included in the resonators.        
Moreover, Japanese Unexamined Patent Application Publication No. 10-65490 discloses a SAW (Surface Acoustic Wave) band rejection filter including a plurality of series arm resonators and a plurality of parallel arm resonators to which respective inductance elements are connected in parallel. In this SAW band rejection filter, resonant frequencies of the parallel arm resonators may be controlled by controlling characteristics of the inductance elements connected to the parallel arm resonators in parallel. Accordingly, a low insertion loss is attained.
As described above, according to the filter device 101 disclosed in Japanese Patent No. 3827232, the sharpness of the filter characteristic at the ends of the passband can be enhanced. However, there arises a problem in that, if the capacitor is connected to the parallel arm resonator in parallel as disclosed in Japanese Patent No. 3827232, an area in which the ladder circuit occupies is increased, and accordingly, cost for fabrication of the filter device is increased.
Furthermore, in a case where different electromechanical coupling coefficients are obtained by setting different standard film thicknesses (h/λ) of IDTs included in resonators, different thin-film forming processes should be performed for different resonators. Accordingly, cost for fabrication of the filter device is increased.
Moreover, an electromechanical coupling coefficient is mainly determined depending on a wafer member, and does not considerably depend on a duty ratio. Therefore, it is difficult to obtain different electromechanical coupling coefficients of resonators by setting different duty ratios of IDTs included in the resonators. Accordingly, it is difficult to efficiently improve the sharpness of a filter characteristic at ends of a passband.
As disclosed in Japanese Unexamined Patent Application Publication No. 10-65490, impedance characteristics of resonators can be controlled by controlling characteristics of inductance elements connected to parallel arm resonators in parallel. Accordingly, sharpness of a filter characteristic at ends of a passband can be enhanced. However, in a case where the characteristics of the inductance elements connected to the parallel arm resonators in parallel are controlled, since electromechanical coupling coefficients are not substantially changed, it is difficult to enhance the sharpness of the filter characteristic at the ends of the passband while an attenuation amount is maintained.